As is well known to those of skill in the art, passive and hybrid microelectronic circuit components (hereinafter circuit “components”), are fabricated in an array on or in the interior of a ceramic substrate. The ceramic substrate is cut, sometimes called diced, to singulate the circuit components from one another.
For the past 30 years, the predominant method of singulating ceramic substrates involved using a pulsed CO2 laser dicing process in which a pulsed laser was aligned with and then directed along a street to form a “post hole” scribe line. FIG. 1 is a scanning electron micrograph (SEM) of a post hole scribe line 2 formed by pulsed CO2 laser cutting. As shown in FIG. 1, post hole scribe line 2 includes spaced-apart shallow vias 4 that extend into the thickness of a ceramic substrate 6 along the length of scribe line 2. Following formation of the post hole scribe line, force is applied to the ceramic substrate portions on either side of the scribe line to effect breakage of the ceramic substrate into separate pieces.
Although pulsed CO2 laser cutting offers advantages in speed, cleanliness, accuracy, and reduced kerf, the use of the post hole scribe line creates separate ceramic pieces having jagged and uneven side edges as well as significant melted slag residue. As shown in the SEM of FIG. 2, ceramic substrate piece 6 formed in accordance with the post hole scribe line method has sinusoidal-shaped side edges 8 rather than the preferred straight and smooth side edges. Further, ceramic substrate piece 6 includes slag residue 7.
Pulsed CO2 laser cutting also leads to distortion of the interior structure of the ceramic surface, resulting in structurally weak components. Specifically, the strength of the ceramic substrate is reduced, decreasing its ability to withstand thermal or mechanical stress. The structural weakness of the interior often evidences itself in an increased number of microcracks present near the laser scribe line. FIGS. 3A and 3B are SEMs showing cross-sections of ceramic substrate pieces formed using pulsed CO2 laser cutting. FIG. 3A shows a ceramic substrate piece at 10× magnification, and FIG. 3B shows the side edge of a ceramic substrate piece at 65× magnification. Both figures show multiple microcracks 9 extending from side edge 8 into the interior of the ceramic substrate piece 6. According to Weibull's strength theory, the flexural strength of the ceramic substrate decreases as the density of microcracks increases (Weibull, W., Proc. Roy. Swedish Inst. Engrg. Research, 193.151 (1939)). Manufacturing costs increased because many of the circuit components were discarded as a consequence of their insufficient flexural strength.
Until recently, fired ceramic substrates had length and width dimensions of about 6×8 inches and a thickness of about 1 mm. The uneven side edges, slag residue, and microcracks formed as a result of pulsed CO2 laser cutting were tolerable when scribing ceramic substrates having these specifications.
However, recent technological advances in component miniaturization necessitate singulation of circuit components having length and width dimensions of about 1 mm×0.5 mm (0402) or 0.5 mm×0.25 mm (0201) and a thickness of between about 80 microns and about 300 microns. Circuit components of this density and/or thickness cannot tolerate such uneven side edges, slag residue, and microcracks resulting from either pulsed CO2 or ND:YAG laser cutting because these methods of laser cutting adversely affect the specified circuit component values and/or subsequent component processing.
One prior art attempt to singulate these smaller and thinner circuit components entailed sawing through the ceramic substrate using a saw blade that had been aligned with a “street” created by the thick and thin film patterns formed on or in the interior of the ceramic substrate as part of the process of forming the circuit components. Alignment of the saw blade and street was achieved using an alignment system. Tape was preferably attached to the ceramic substrate before sawing to provide support for the singulated circuit components upon completion of sawing. Problems with this prior art method include inexact positioning and alignment of the saw blade, mechanical wobbling of the saw blade, and uneven or rough surfaces resulting from the mechanical nature of cutting with a saw blade. Further, the width of the scribe line had to be sufficiently large to accommodate the width of the saw blade. A typical saw blade is 75-150 microns wide along its cutting axis, producing cuts that are about 150 microns wide. Because the resulting scribe lines had relatively large widths and therefore occupied a greater portion of substrate surface, fewer components could be produced for any given size of ceramic substrate. This resulted in more wasted surface area, less surface area available for circuit component parts, and a greater than optimal cost of each circuit component.
The method by which most large-sized chip resistor components are formed involves initially precasting the scribe lines into a ceramic substrate in an unfired state. The resistor components are then printed on the fired ceramic substrate, and the substrate is broken along the scribe lines to form separate circuit components.
For smaller circuit components, a YAG laser is used to form the scribe lines in a fired ceramic substrate. These scribe lines are used to align subsequent printing steps. However, YAG laser scribing is slow and does not provide the desired vertical breaks. An ultraviolet (UV) YAG laser may replace the YAG laser, yielding much higher scribe speeds and better breaks. However, as circuit component size further decreases, use of this method became untenable because the circuit components were of such a small size that it became impossible to align the printing patterns to the previously formed scribe lines.
It consequently became necessary to form off-axis scribe lines. This need was also evident for ceramic components (chip capacitors, conductors, filters, etc.) that had been fired, a process that entails exposing the ceramic substrate to temperatures of between about 750° C. and about 1100° C. Prolonged exposure to these high temperatures causes the ceramic substrates to warp along one or both axis, resulting in the formation of a non-standard shaped ceramic substrate. Thus, a need arose for a laser that could align with and accurately scribe these nonstandard-shaped ceramic substrates to form multiple nominally identical circuit components. Those skilled in the art will understand that the printing and scribing sequence can be interchanged without affecting the end result.
Additionally, many circuit components have a top layer that includes metal. This layer can extend into either or both of the streets extending along the x-axis or the y-axis. Those of ordinary skill will readily recognize that the existence of metal in the top layer prevents the use of a CO2 laser since the metal reflects the CO2 laser beam. Further, mechanically sawing a metal-containing layer is undesirable because the ductile nature of many metals, such as copper, make mechanical sawing of a metal-containing layer an extremely slow and difficult process.
Via drilling using an UV YAG laser has been used extensively in the printed wiring board (PWB) industry. Specifically, a UV-YAG laser emits a laser beam that cuts through the top, metal-containing layer before the underlying organic material is drilled. Thus UV laser drilling of copper, and other metals used in the fabrication of circuit components, is well understood by those of ordinary skill in the art.
What is needed, therefore, is an economical method of forming a scribe line in a ceramic substrate that facilitates the clean breakage of the ceramic substrate into separate circuit component parts having clearly defined side margins, minimal slag residue, and a reduced incidence of microcracking.